There is shown in JP Patent Kokoku Publication No. 63-1278 a conventional silicon nitride sintered body having a surface coating of a ceramic material having high hardness or superior wear resistance.
A further silicon nitride sintered body has been known in which crystal phases of both .alpha.- and .beta.-Sialons are homogeneously distributed over the entire sintered body (JP Patent Kokoku Publication No. 63-35594 (1988) etc.). Problems to be solved by the present invention:
As for the former sintered body, it is difficult to obtain a sufficient bonding force to the substrate through the difference in thermal expansion and/or chemical affinity between the coating ceramic material and the silicon nitride substrate, entailing a high cost counted as a problem in economy. Thus the former sintered body has found almost no practical application.
The latter sintered body which includes both the crystal phases of .alpha.- and .beta.-Sialons evenly distributed over the entire sintered body suffers variation in its properties as their proportion changes and cannot develop every property. Namely, if one attempts to enhance one property any of other properties is forced to be deteriorated, resulting in at most an intermediate performance on balance of the both.